Do you want to learn about GIS or about specific softwares/languages? What would you say your level of fundamental GIS knowledge is?
ESRI software is a tool, one of many. It can be used to teach GIS, just as QGIS or many others could. The GIS certficate program I completed used ESRI software for any number of reasons. However actual instruction and education was primarily aimed at GIS, not ESRI - meaning it covers concepts like Intersect and not just tools like the specific Intersect tool in ESRI software. Another example would be there are many syntaxes and functions available for SQL that differ from database to database. My course used Access to explain the concepts, but pointed out that things did vary and you can do some things with one you may not be able to do with another.
It's possible that the course you are looking at is 'software training' and not an 'education'. But it's also possible the course just uses that software for illustration or hands-on experience with the concepts taught. If you're knowledgable and comfortable with fundamental GIS skills and abilities, then self-directed study for specific areas of interest (ie Python, or just PostgreSQL, or QGIS software) may be the best approach. Otherwise a more general GIS education may be helpful. Your undergrad degree surely covered some of that, but maybe not all of it.
Without more info on the program it's hard to make any specific suggestion, but as GISKid said, if it's free you don't have a lot to lose and you should get something out of it.
Also, note the Related question links on the right - there are many similar and related questions here already you may want to review.